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Targeting ordered oligothiophene fibers with enhanced functional properties
by interplay of self-assembly and wet lithography†
Denis Gentili,*a Francesca Di Maria,b Fabiola Liscio,c Laura Ferlauto,c Francesca Leonardi,a Lucia Maini,d
Massimo Gazzano,b Silvia Milita,c Giovanna Barbarellab and Massimiliano Cavallini*a
Received 20th June 2012, Accepted 23rd August 2012
DOI: 10.1039/c2jm33998f
Reproducible spatial control of a self-assembly process of fiberforming oligothiophenes was achieved by using confinement effects.
This strategy allowed the direct integration with a precise control
over density, orientation, and size of supramolecular semiconducting
fibers in OFET devices, demonstrating that well-aligned fibers
exhibit a substantial enhancement of electrical performances.
Achieving the full control of supramolecular self-assembly through
the noncovalent interaction of p-conjugated organic functional
materials in well-defined and ordered superstructures is a key issue for
the technological application of supramolecular chemistry. In
particular, this is crucial for applications in large areas and flexible
electronic devices of organic charge-transport materials since their
electronic properties depend on both the chemical structure and the
molecular packing/orientation.1–5 Functional supramolecular nanoand microsized fibers6,7 can be used as interconnecting modules in
integrated molecular circuits or as active layers in organic devices
such as field-effect transistors and solar cells.8–10 In particular, fibers
can facilitate charge carrier mobility through long-range molecular
orientation with respect to device electrodes.11
Organic field-effect transistors (OFETs) based on a single fiber12–14
or bundles of fibers,15,16 with excellent charge mobilities in the former
case, have been reported. However, the application of conducting
fibers is limited by the difficulty of controlling morphological
parameters, such as fiber distribution and density (i.e. number of
fibers per unit area), which affect the reproducibility and the
a
Consiglio Nazionale delle Ricerche, Istituto per lo Studio dei Materiali
Nanostrutturati (CNR-ISMN), via P. Gobetti 101, 40129 Bologna,
Italy. E-mail: [email protected]; [email protected]; Fax:
+39 051 6398516; Tel: +39 051 6398522
b
Consiglio Nazionale delle Ricerche, Istituto per la Sintesi Organica e la
Fotoreattivit
a (CNR-ISOF) (MG, GB) and Laboratorio MIST.E-R
(FDM), via P. Gobetti 101, 40129 Bologna, Italy
c
Consiglio Nazionale delle Ricerche, Istituto per la Microelettronica e
Microsistemi (CNR-IMM), via P. Gobetti 101, 40129 Bologna, Italy
d
Dipartimento di Chimica G. Ciamician, Universit
a degli Studi di Bologna,
via Selmi 2, 40126 Bologna, Italy
† Electronic supplementary information (ESI) available: Experimental
procedures, fibers alignment process, optical microscope images, AFM
images, and 2DGIXD and single crystal data. CCDC 876823. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2jm33998f
20852 | J. Mater. Chem., 2012, 22, 20852–20856
performance of devices. Alignment of fibers, also used in OFET
devices, was successfully obtained by several techniques, such as dipcoating,17 filtration-and-transfer,18 by electric19,20 and magnetic
fields,21,22 or using molecular gelators.23,24 Although very efficient,
these approaches suffer from the lack of precise control over size,
placement (preventing the overlap), and density of the fibers that,
together with an appropriate molecular packing25 and a proper fiber
alignment, should ensure reproducible properties as required for
device applications. Therefore, a technology able to directly integrate
organic semiconducting fibers with control over size, orientation,
position and density on devices is highly desirable. Recently, wet
lithographic methods have been used to fabricate OFETs based on
organic semiconducting nanostripes with improved charge transport
properties.26,27 Although these methods have permitted the control of
stripes distribution and the enhancement of crystalline structures,
they have never been used with supramolecular fibers.
Here, we demonstrate for the first time that, by combining the
spontaneous self-assembly of fiber-forming oligothiophenes with
easy-to-handle and low cost wet lithographic techniques,28 transistors
based on bundles of semiconducting well-aligned supramolecular
fibers can be achieved, avoiding mechanical manipulations and postfabrication processes for their integration into devices. We have taken
advantage of the high plasticity of oligothiophene compounds and
their self-organization capability in a confined system.29 In addition,
we show that moving from random- to well-aligned fibers there is an
impressive enhancement of electrical performances, which can reach
up to 3 orders of magnitude for charge carrier mobility.
We have chosen sulphur over-rich thiophene octamers 1–3
(Fig. 1a), recently reported by Di Maria et al.,30 which self-assemble
into supramolecular nano- and micrometer sized, thermodynamically
very stable, crystalline 1–3 fibers, respectively, when exposed to vapor
of a non-solvent. These fibers, whose morphologies are univocally
determined by the molecular structure and therefore can be reproducibly generated on various types of surfaces, exhibit high crystallinity, strong fluorescence, and semiconducting properties.
Non-solvent vapor-induced crystallization in combination with
lithographically controlled wetting (LCW),31 which has already
proven to be a powerful tool for manipulations of soluble
organic26,27,32 and metal–organic compounds,33–35 has been used
herein to control the self-assembly process of compounds 1–3
(Fig. 1b). In an airtight container saturated with non-solvent vapor,
an elastomeric stamp, consisting of parallel lines (pitch ¼ 1.6 mm, full
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Fig. 1 (a) Molecular structures of oligothiophenes 1–3. (b) Schematic
representation of non-solvent vapor-induced crystallization in combination with lithographically controlled wetting (LCW).
width at half-maximum FWHM ¼ 1 mm, and 220 nm deep), was
placed in contact with a 1–3 solution film spread on a SiO2/Si
substrate with or without gold electrodes (see ESI†). As shown in
detail in Fig. 1b, there is a confinement effect due to the capillary
forces that pin the solution under stamp protrusions. At the same
time, the self-assembly process induced by non-solvent vapor takes
place and leads to the formation of a well-oriented array of fibers
imposed by the motif of the stamp protrusions.
The method whose detailed protocol is described in ref. 28a is
highly reproducible with a success rate higher than 90% (i.e.
considering as a successful case a sample containing aligned fibers in
areas larger than 2 2 mm2). On the other hand, without using the
elastomeric stamp, randomly distributed crystalline fibers of 1–3 were
formed on the surface. Fibers were characterized by atomic force
microscopy (AFM), optical microscopy, and by grazing incidence
X-ray diffraction (GIXD). We have fabricated at least 12 different
samples for each compound.
Optical images (Fig. 2a and b and S1†) show the successful
fabrication of well-aligned 1–3 fibers and reveal that the stamp motif
has imposed the fibers density and orientation, as shown by the fact
that where the stamp ends the fibers exhibit a pronounced sagging
(Fig. 2a and b). All aligned fibers showed birefringence that extinguishes upon a 45 rotation with respect to the polarizers (Fig. 2a,
inset), indicating that the fibers have a pronounced crystalline
directional order. Optical images further confirm that our technique
allows the fabrication of OFETs in a bottom contact configuration
with 1–3 fibers aligned perpendicularly to the gold electrodes
(Fig. 2c, S2 and S3†). On the other hand, without stamp,
fibers result randomly distributed within entangled bundles (Fig. 2d,
S4 and S5†).
This journal is ª The Royal Society of Chemistry 2012
Fig. 2 Optical micrographs collected under (a) cross-polarized light
oriented along the axes of the image, inset: after rotation of 45 (scale
bar ¼ 100 mm), and (b) unpolarized light (scale bar ¼ 50 mm) of aligned 1
fibers on a SiO2/Si surface. Optical micrographs collected under (c) crosspolarized light oriented along the axes of the image of aligned fibers (scale
bar ¼ 50 mm), inset: unpolarized light (scale bar ¼ 10 mm), and (d)
randomly distributed (scale bar ¼ 50 mm) 1 fibers on an interdigitated
gold electrode/SiO2 surface.
Fig. 3 Typical AFM topography (scale bar ¼ 20 mm, z scale 0–250 nm),
and corresponding profiles of (a and b) aligned and (c and d) randomly
distributed fibers on an interdigitated electrode/SiO2 surface (scale bar ¼
20 mm, z scales (a) 0–250 nm, (b) 0–500 nm). The inset in (a) shows the
AFM topography image of a PDMS stamp (scale bar ¼ 10 mm, z scale 0–
250 nm). Here we have shown fibers of 1 (2 and 3 exhibit a similar
morphology).
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AFM analysis (Fig. 3a–d, S6 and S7†) reveals that the role of the
stamp (Fig. 3a, inset) is not limited to imposing the fibers orientation
and density, but the confinement effect of the stamp also controls the
size of the fibers. Independent of the oligothiophene used, narrow
distributions of the FWHM (1.1 0.2 mm) and heights (159 20 nm) were found for aligned fibers. Noteworthy, AFM profiles
(Fig. 3b) indicate that the discontinuous nature of the substrate used
for OFET fabrication (150 nm high interdigitated gold electrodes
preformed on a thermal SiO2 surface) does not affect the morphology
of the fibers whose lengths reach up to hundreds of mm, achieving a
good interconnection between the source and drain electrodes.
Deeper insight into the molecular organization of 1–3 fibers was
obtained by 2D-GIXD analysis. All samples with randomly distributed fibers showed diffraction images that contain a large number of
reflections, which are invariant upon rotation of the sample around
its normal (Fig. S8†). These results display the crystalline nature of
the fibers and the isotropic distribution of the crystals that reflects the
random distribution of the fibers. In contrast, 2D-GIXD analysis of
aligned 1–3 fibers reveals different reflections when the X-ray beam is
parallel or perpendicular to their direction (Fig. 4a and b, S9 and
S10†). This strong anisotropy of distribution of the reflections indicates that the alignment of the fibers involves a pronounced anisotropic distribution of their crystalline structure, in agreement with
what was previously viewed with the optical microscope under crosspolarized light. The crystalline structure of 1, preliminarily determined by single crystal diffraction (see ESI†), enables us to establish,
from the 2D-GIXD analysis, the exact orientation of the molecule
inside the fiber. In both the beam directions, the (010) spot lies along
the vertical line, i.e. (010) planes are parallel to the substrate surface,
which indicates a molecular ‘‘edge-on configuration’’ with the oligothiophene backbone lying almost perpendicularly to the substrate
surface (Fig. S11†). When the X-ray beam is parallel to the fiber only
a few reflections, i.e. (011), (021), (031) and (023), are recorded,
whereas when the X-ray beam is perpendicular these reflections
disappear and others, namely (130), (131) and (142), are recorded.
This indicates that molecules self-assemble along the a direction by
facing each other as shown in Fig. 4c. Analysis of the (010) arc width
provides further structural information. When the X-ray beam is
parallel to the fibers’ direction, the (010) arc width results larger,
revealing a larger (010) misorientation that can be ascribed to a
relative tilt of molecules belonging to adjacent fibers. Moreover, a
smaller contribution of a pronounced misoriented plane can be
related to the helical folding occurring along the fibers’ direction.
These effects are not detected in the diffraction recorded with the
beam perpendicular to the fibers’ direction. Similar features, also with
a more pronounced anisotropy, have been recorded for aligned 2 and
3 fibers (Fig. S9 and S10†), while single crystal diffraction measurements for both octamers are under investigation.
In order to perform the electrical characterization, OFETs were
built in a bottom-gate, bottom contact architecture: fibers were
directly grown on interdigitated gold source and drain microelectrodes prefabricated on a thermal SiO2 surface (see ESI†). Saturated
charge mobilities msat were measured in air with a drain voltage VD ¼
40 V and calculated by estimating the coverage, and therefore the
effective channel widths, on the basis of optical microscopy images.
Table 1 summarizes the electrical performances of the OFETs based
on random (entries 1–3) and aligned (entries 4–6) 1–3 fibers.
With randomly distributed fibers (entries 1–3), no field-effect
characteristics were observed for 2-based OFETs, while 1- and 3based OFETs showed p-channel behavior. Although occasionally
higher values of msat were observed (up to one order of magnitude
higher), the mean values of electrical characteristics for devices based
on randomly distributed fibers are very poor (see Table 1).
On the other hand, all aligned fibers, including those of compound
2, clearly displayed p-channel field-effect characteristics (entries 4–6).
As a result of alignment, for all compounds, msat amounts to more
than 104 cm2 V1 s1 that, for fibers 1 and 3, corresponds to an
increase of 2 orders of magnitude. Moreover, also considerable
improvements of both threshold voltage VT and ION/IOFF ratio were
observed. Further improvements of the electrical performances were
achieved by chemical functionalization of a gate dielectric with
octadecyltrichlorosilane (OTS, see ESI†). The electrical transfer and
output characteristics of OFETs with aligned 1–3 fibers on OTS are
displayed in Fig. 5. In this bias range, the output curves do not exhibit
saturation due to the high positive threshold voltage. All aligned
fibers exhibit msat increased by about 3 orders of magnitude in
comparison with OFETs based on randomly distributed fibers.
Furthermore, the VT values are decreased down to 22–26 V, and the
ION/IOFF ratios are increased by about one order of magnitude
(Table 1), except in the case of 2, which exhibits an unexpected small
decrease of ION/IOFF whose explanation is under investigation.
Surprisingly, no measurable field effect was observed for randomly
distributed fibers on OTS, and no relevant effects on the fiber
Table 1 Electrical characteristics of bottom-gate, bottom contact
OFETs based on randomly distributed and aligned 1–3 fibers
Fig. 4 2D-GIXD images of aligned 1 fibers recorded with the X-ray
beam (a) parallel and (b) perpendicular to their direction. (c) Schematic
illustration of the molecular organization of oligothiophene 1 along the
fiber direction (red arrow).
20854 | J. Mater. Chem., 2012, 22, 20852–20856
Entry
Fiber
Substrate
msat/cm2 V1 s1
VT/V
ION/IOFF
1
2
3
4
5
6
7
8
9
1a
2a
3a
1b
2b
3b
1b
2b
3b
SiO2
SiO2
SiO2
SiO2
SiO2
SiO2
SiO2/OTS
SiO2/OTS
SiO2/OTS
1.38 106
—
1.15 106
4.05 104
6.72 104
3.68 104
3.02 103
1.02 103
6.24 104
288
—
199
77
53
44
26
22
22
2
—
2
4
22
25
23
11
83
a
Random fibers. b Aligned fibers.
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6
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7
8
9
10
11
12
13
14
15
Fig. 5 Transfer at VD ¼ 40 V and output curves at various gate
voltages VG corresponding to OFETs functionalized with OTS and based
on aligned fibers of: (a and b) 1; (c and d) 2; and (e and f) 3.
16
17
18
properties were observed by changing the fiber size in the range of
350–750 nm and the pitch in the range of 1600–500 nm.
In summary, we have reported the first example of controlled selfassembly of fiber-forming oligothiophenes by confinement effects.
This strategy allowed the direct integration of well-ordered supramolecular semiconducting fibers in OFET devices, which has
meaningfully improved their electrical properties. Although
compounds 1–3 have a low charge mobility when compared to a thin
film of other oligothiophenes36 (which is due to their molecular
structure and the different molecular packing in the ‘‘aggregate’’
phase), our study opens the way to the reproducible fabrication of
functional devices that requires controlled integration of supramolecular fibers with precise density, orientation, and size. Further work
will be focused on reducing the size of the printed fibers down to the
nanoscale and the extension of the study to many other fibers. This
will lead to the development of a new generation of devices based on a
variety of multifunctional fibers with possible applications in several
fields of technology.
19
20
21
22
23
24
25
26
Acknowledgements
27
This work was supported by the national project PRIN prot.
2009N9N8RX_003.
28
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